The present invention relates generally to tooling used to manufacture lightweight fan blades used in gas turbine engines.
Gas turbines include, but are not limited to, gas turbine power generation equipment and gas turbine aircraft engines. A gas turbine includes a core engine having a high pressure compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the high pressure compressor. A typical front fan gas turbine aircraft engine adds a low pressure turbine (located aft of the high pressure turbine) which is connected by a smaller diameter coaxial shaft to drive a front fan (located forward of the high pressure compressor) and to drive an optional low pressure compressor (located between the front fan and the high pressure compressor). The low pressure compressor sometimes is called a booster compressor or simply a booster.
The fan and the high and low pressure compressors and turbines have airfoils each including an airfoil portion attached to a shank portion. Rotor blades are those airfoils which are attached to a rotating gas turbine rotor disc. Stator vanes are stationary airfoils which are attached to a non-rotating gas turbine stator casing. Typically, there are alternating circumferential rows of radially-outwardly extending rotor blades and radially-inwardly extending stator vanes. When present, a first and/or last row of stator vanes (also called inlet and outlet guide vanes) may have their radially-inward ends also attached to a non-rotating gas turbine stator casing. Counter-rotating xe2x80x9cstatorxe2x80x9d vanes are also known.
Conventional airfoil designs used in the compressor section at the engine typically have airfoil portions that are made entirely of metal, such as titanium, or are made entirely of a composite. A xe2x80x9ccompositexe2x80x9d is defined to be a material having any (metal or non-metal) fiber filament embedded in any (metal or non-metal) matrix binder, but the term xe2x80x9ccompositexe2x80x9d does not include a metal fiber embedded in a metal matrix. The term xe2x80x9cmetalxe2x80x9d includes alloys such as titanium Alloy 6-2-4-2. An example of a composite is a material having graphite filaments embedded in an epoxy resin.
The all-metal blades, including costly wide-chord hollow blades, are heavier in weight which results in lower fuel performance and require sturdier blade attachments, while the lighter all-composite blades are more susceptible to damage from bird ingestion events. Known hybrid blades include a composite blade having an airfoil shape which is covered by a surface cladding (with only the blade tip and the leading and trailing edge portions of the surface cladding comprising a metal) for erosion and foreign object impacts. The fan blades typically are the largest (and therefore the heaviest) blades in a gas turbine aircraft engine, and the front fan blades are usually the first to be impacted by foreign objects such as birds.
Recent improvements have resulted in lighter-weight gas turbine blades, and especially a gas turbine aircraft engine fan blade that is comprised of a combination of metal and lightweight materials. These blades have been made lighter by removing metal from the pressure side of the blade. In order to retain the aerodynamic characteristics of the blade, the removed metal is replaced by the lightweight material. Restoring the aerodynamic characteristics to the blade by adding the lightweight material to replace the removed metal involves the use of specialized tooling. However, the specialized tooling that includes a special caul sheet currently used in the process of adding lightweight material to the pressure side of the fan blade in order to restore aerodynamic characteristics requires that an effective seal be formed against the blade pressure side by the caul sheet. The caul sheet currently used in the process of adding the lightweight material relies on an O-ring to form the seal with the pressure side of the blade. However, the O-ring can cause the caul sheet to stand off from the pressure side of the blade. The result is that there is a lack of good contact between the caul sheet and the pressure surface, and a step is formed in the molded surface of the lightweight material that can rise above the pressure surface up to the diameter of the O-ring. This step is undesirable, as it adversely affects the aerodynamics of the pressure side of the blade. It is time consuming to and very difficult to remove this step from the lightweight material, as the material is also very tough. What is needed is better method using improved tooling for adding lightweight material to a blade.
A flexible tool is formed to fit over the pressure or concave side of a metallic airfoil that includes a lightweight material component for a gas turbine engine during fabrication of the airfoil. Typically the airfoil is a metallic fan blade. The metallic fan blade includes pockets or cavities that have been machined into the blade in order to reduce the weight of the blade. The tool is a flexible body manufactured from sheets of composite material and includes an integral elastomeric seal.
The flexible tool is formed by laying up thin sheets of composite material that includes fiber over a metallic master tool. As used herein, composite material is material formed from sheets of plastic resin matrix material having a fiber reinforcement, in which the fiber reinforcement may be unidirectional or bidirectional (woven). This material is sometimes referred to as prepreg. The metallic master tool has a profile that matches the profile of the pressure side of the fan blade, but includes a plurality of slots that are located at positions that correspond to locations along the perimeter of the fan blade, that is, positions just beyond the leading edge, trailing edge or tip end. As used herein, matching the profile of the pressure side of the fan blade means that the metallic tool has a surface that substantially corresponds to the contours, dimensions and curvatures as the pressure side of a corresponding metallic fan blade that is manufactured without cavities. The slot positions correspond to preselected positions, which allow the flexible caul sheet and seal to be correctly assembled to the blade. The slot depth may vary, but need only be sufficiently deep to allow the layers of composite material to be laid into them, thus forming lugs that positively locate the flexible caul sheet when it is placed on to the concave (pressure) side of the blade.
The elastomeric material is partially cured and is placed along the tool within an area inside the outline of the blade, which is permanently marked on the tool, such as by scribing the tool surface. Thus, the elastomeric material is placed on the tool inside of markings that correspond to the perimeter of the blade. The sheets of composite material are laid up to achieve a predetermined thickness over the elastomeric material, over the tool surface in the region outlining the blade and into the slots on the tooling surface. The predetermined thickness provides a predetermined stiffness so that the flexible tool will not deform when lightweight a material is injected under pressure into the pockets of the blade beneath the tool. The tool also includes at least one injection port corresponding to a pocket or cavity so that the lightweight material can be injected through the tool into the blade pockets. Additional ports, each corresponding to a pocket, may be added as required. A surround frame for added local stiffness is assembled from sheets of composite material and is separated from the flexible tool using a TEFLON(copyright) (polytetrafluoroethylenexe2x80x94PTFE) film. The surround frame extends around the perimeter of the blade outline on the tooling surface so that it overlies the partially cured elastomer and the sheets of composite material.
The metallic master tool with the partially cured elastomer, the laid up composite sheets and the surround frame secured thereto is then placed in an elevated temperature atmosphere under pressure to cure the composite sheets and the elastomer to form the flexible tool. After curing, the surround frame is removed from the flexible tool, which in turn is removed from the metallic master tool. The flexible tool, which now includes an integral seal extending around its perimeter formed as the partially cured elastomer cures with the composite sheet, has a profile that matches the profile of the pressure side of the blade and can now be used to facilitate the injection of lightweight material into pockets of a fan blade by positioning the flexible tool over the fan blade and securing it into position. The integral seal is concave at room temperature to facilitate assembly of the tool to the blade, but expands on heating to form an effective seal against the blade.
An advantage of the present invention is that the problem of standoff is eliminated. Standoff, which is caused by use of an O-ring, results in poor contact between the tool and the blade, and results in a step when a flowable, curable, lightweight material is injected into the blade pockets.
Another advantage of the present invention is that a plurality of identical flexible tools can be manufactured from the metallic master tool that has an indefinite life.
Another advantage of the present invention is that the tool is more easily located on the blade, as the integral seal is concave at room temperature and the problems associated with positioning a tool having a movable O-ring that extends away from the tool surface are eliminated.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.